As semiconductor manufacturers move toward fabricating smaller features on a wafer, a photomask assembly that is used to project the features onto the wafer has become increasingly important. For example, features on the wafer are becoming smaller than the wavelength of light used to print the features. At shorter wavelengths, the light in a photolithography system has more energy and can be destructive to a photomask assembly manufactured with conventional materials.
A conventional photomask assembly generally includes a photomask, also known as a reticle or mask, and at least one pellicle that covers the patterned side of the photomask. A standard photomask includes a patterned layer of opaque or partially transmissive material formed on a transparent substrate. A pellicle typically includes a thin film attached to a frame. The thin film acts as a cover that keeps contaminants off the surface of the photomask during a lithography process. The height of the frame corresponds to a distance from the surface of the photomask such that when a contaminant lands on the thin film, it is out of focus and is not imaged onto the wafer. The pellicle frame is typically mounted on the photomask with an annular shaped adhesive gasket that is attached to the bottom and around the perimeter of the pellicle frame. The gasket is typically made of an adhesive material and functions as an attachment device as well as a seal.
At exposure wavelengths in the deep ultra violet (DUV), vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) ranges, various characteristics of the photomask assembly are of concern. For example, the flatness of a photomask becomes more critical as the exposure wavelength decreases. Existing pellicle application techniques use pressure to create an adhesive bond between the pellicle frame, the gasket and the transparent substrate. During mounting, a pressure is applied to the pellicle in order to create a seal between the pellicle and the photomask. The pressure distorts both the pellicle frame and the photomask substrate. When the pressure is removed, the pellicle frame attempts to return to its original shape, which may cause photomask distortion. The photomask assembly eventually reaches an equilibrium, but the frame and photomask remain in a stressed state. Over time, the stress may cause the flatness of the photomask to degrade and cause registration errors on the wafer. Flatness may also be affected by an inherent stress that is present in a material that is deposited on the transparent substrate and used to create the patterned layer. This stress, in addition to any stress caused by the pellicle, may eventually warp the photomask and cause registration errors during a lithography process.
Furthermore, conventional adhesive materials used for the gasket may embrittle and outgas with prolonged exposure to electromagnetic energy with wavelengths below approximately 250 nanometers (nm). The outgassing may cause the seal provided by the gasket to degrade. If the seal is broken, contaminants in the lithography system may reach the surface of the photomask and cause defects to appear on the wafer.
The materials used to form the various parts of the photomask assembly are also critical in lithography systems that use exposure wavelengths of less than approximately 250 nm. For example, a pellicle film made of nitrocellulose can degrade and therefore, cause the photomask to be contaminated during the lithography process.